1. Technical Field
The present invention relates to an acceleration sensor and an acceleration detection apparatus, and particularly to an acceleration sensor that not only converts the direction of a force produced when acceleration is applied into another direction but also increases the magnitude of the force, and an acceleration detection apparatus.
2. Related Art
An acceleration sensor using a piezoelectric resonator detects acceleration applied to the acceleration sensor based on a change in resonance frequency of the piezoelectric resonator that occurs when a force acts thereon in the direction of a detection axis thereof.
Japanese Patent No. 2,851,566 discloses an accelerometer having a double-ended tuning fork piezoelectric resonator attached to opposing corners of a parallelogram frame and configured to receive a compressive or tensile force acting on the other opposing corners and a method for manufacturing the accelerometer.
The accelerometer of the related art (Japanese Patent No. 2,851,566) will be described with reference to the drawings. FIG. 6 is a diagrammatic cross-sectional view showing a schematic configuration of the accelerometer of the related art. FIGS. 7A and 7B are diagrammatic views showing the configuration of a central device of the accelerometer of the related art. FIG. 7A is a plan view, and FIG. 7B is a cross-sectional view. FIG. 8 is a diagrammatic plan view showing the configuration of a transducer device of the accelerometer of the related art.
The accelerometer has a mass portion 116 connected to a support 117 by a flexure 118 for movement along a sensitive axis (detection axis) 119, as shown in FIG. 6. A pair of force sensing crystals 121 and 122 connected between the mass portion 116 and the support 117 vibrate at frequencies (resonance frequencies) corresponding to forces applied thereto. Frequency oscillators 123 and 124 excite the force sensing crystals 121 and 122, and signals from the frequency oscillators 123 and 124 are inputted to a summation circuit 126, which provides an output signal corresponding to the difference in frequency between the two signals.
The accelerometer is formed of five disc-shaped elements made, for example, of quartz crystal (crystalline quartz) and stacked together along the sensitive axis 119. That is, the accelerometer includes a central device 127 shown in FIGS. 7A and 7B, a pair of transducer devices 128 shown in FIG. 8 on opposite sides of the central device 127, and a pair of lids (not shown) on the outer sides of the transducer devices 128.
The central device 127 includes a fixed portion 134 and a movable portion (seismic mass) 133 having a mass, as shown in FIGS. 7A and 7B. The movable portion 133 is hingedly attached to the fixed portion 134 by a pair of flexures 136 for movement about a hinge axis 137 that extends in the direction perpendicular to the sensitive axis 119 (see FIG. 6). The movable portion 133 and the fixed portion 134 are disposed within a mounting ring 139 to which the fixed portion 134 is attached. An isolating ring 141 is disposed outside and concentrically about the mounting ring 139 with flexible arms 142 connecting the mounting ring 139 to the isolating ring 141. The central device 127 is formed as a unitary structure formed of the components described above.
Each of the transducer devices 128 includes a mounting ring 146 within which a force sensing device (force sensing crystal) 147 and a coupling plate 148 are disposed, as shown in FIG. 8. The force sensing device 147 has a double-ended tuning fork piezoelectric resonator 151 connected to opposing corners of a parallelogram frame 149 formed of four links 152 with pads 154 and 156 disposed on the other opposing corners. One of the pads, the pad 154, is formed integrally with the coupling plate 148, and the other one of the pads, the pad 156, is formed integrally with the mounting ring 146.
The coupling plates 148 of the two transducer devices 128 are connected to major surfaces 138 of the movable portion 133 of the central device 127 shown in FIGS. 7A and 7B with an adhesive, and the mounting rings 146 of the transducer devices 128 are connected to the mounting ring 139 of the central device 127 with an adhesive.
Each of the two lids (not shown) has a circular shape having a recess formed on one side thereof and not only forms a sealed structure but also functions as a damping plate with a gas filled therein. The recesses face the transducer devices 128, and the outer margins of the lids are connected to the mounting rings 146 of the transducer devices 128 with an adhesive.
The accelerometer disclosed in Japanese Patent No. 2,851,566, however, has a problem of a very large number of parts, including the central device 127, the two transducer devices 128, and the two lids. Other problems include a complicated structures of the central device 127 and the transducer devices 128, resulting in an expected low yield of each of the devices, a possible large number of steps of tuning the assembled accelerometer, and a very expensive cost of the accelerometer.
Further, the damping gas sealed in the accelerometer described above degrades the Q factor of the double-ended tuning fork piezoelectric resonator 151 of each of the transducer devices 128, resulting in problems of difficulty in exciting the resonator and decrease in acceleration detection sensitivity.
Moreover, in the accelerometer described above, the movable portion 133, although depending on the shape thereof, disadvantageously resonates with an external vibration source at the time of acceleration detection, resulting in a detection error and degradation in acceleration detection precision and other acceleration detection characteristics.